"Thank you all for helping me through this time with your enormous love & support," the actress tweeted on Monday, along with a picture of them together. "Cory will forever be in my heart."
Monteith, 31, died July 13 in Vancouver, Canada, from a fatal combination of heroin and alcohol. He and Michele confirmed they were an item in early 2012.
Last week, the cast and crew of Glee, including Michele and creator Ryan Murphy, gathered for a private memorial to honor Monteith. Production on the show has been delayed and its season premiere has been moved from Sept. 19 to Sept. 26. The third episode of the season will be a tribute episode to Monteith, after which the show will take a long hiatus.